Holographic display technology is classified into conventional optical holography and computer generated hologram (CGH) that uses a digital computer to simulate, compute and process a variety of optical processes. Conventional optical holography records information of phase and amplitude of object light wave on some kind of media in the form of interference fringes through introducing reference light wave that is coherent with the object light wave to interfere the object light wave according to the principle of optical interference, and then reconstruct the original object light wave to form a 3D image of the original object according to the principle of light diffraction. However, the optical holography technology needs a very stable optical system (non-vibrating and noise-free) and a light source having a high coherence and a high intensity, thereby limiting its application scope notably. In order to resolve the above problems, people begin to research computer generated hologram which uses computers to simulate and compute.
With the computer generated hologram technology, a description mathematical function of object light wave is input into a computer directly to simulate an actual interference process, to compute out interference fringes, to draw a computer generated hologram, and then to put the computer generated hologram into an actual light path to get a reconstructed image. Compared with the conventional optical holography, the computer generated hologram has remarkable characteristics such as low-noise, good repeatability and ability to get hologram of a virtual object, etc.
The processes of generation and reconstruction of computer generated hologram is mainly divided into the following 5 steps. The first step involves sampling, that is, to get a value of an object or wavefront in discrete sampling points. The second step is computation, that is, to compute optical field distribution of object light wave on the hologram plane. The third step is encoding, that is, to encode complex amplitude distribution of the light wave on the hologram plane into transmissivity variation of hologram. The fourth step is mapping, that is, to draw the transmissivity variation into a graph under the control of a computer. If the resolution of the plotting equipment is not high enough, it is possible to draw a relatively large graph first and then get a hologram through a de-scaling process. The fifth step is reconstruction, which is the same as the reconstruction of the optical holography.
After the computer complete encoding of the computer generated hologram, the next step is to display the computer generated hologram in a size and mode which are suitable for optical reconstruction. As the size of each sampling unit in the computer generated hologram is in the order of micron, it needs to use a specialized optical microphotography system or a microlithography system, or a camera if the requirement is relatively low, to microfilm the computer generated hologram which is displayed on a computer screen or printed out on a high resolution photographic film, and then developing and fixing the film to get the computer generated hologram suitable for optical reconstruction. As the recording media is a photographic film, it can only display a static computer generated hologram.
With the development of high resolution electrically-addressed spatial light modulator (SLM) in recent years, amplitude-type or phase-type spatial light modulators having a pixel size in micron scale and the number of pixels over 1 million have become completely practical. The most representative spatial light modulator is a liquid crystal spatial light modulator.
A liquid crystal spatial light modulator is a spatial light modulator fabricated on the base of birefringence effect of liquid crystal molecules; controlled units are independent pixel units and the pixel units are arranged into a one-dimensional or two-dimensional array. Each of the pixel units can independently receive controlling signals such as an optical signal or an electrical signal and the like, and can modulate the input light wave at a pixel order to transform the wavefront of the light wave flexibly.
A light path using a liquid crystal spatial light modulator to realize reconstruction is illustrated as FIG. 1, in which a liquid crystal spatial light modulator 10 is an electrically-addressed liquid crystal spatial light modulator, the liquid crystal spatial light modulator 10 is connected to and receives modulated signals from a computer 11. The computer 11 outputs electrical signals of a computer generated hologram to the liquid crystal spatial light modulator 10. A liquid crystal display (LCD) of the liquid crystal spatial light modulator 10 driven by a driving circuit changes the transmissivity of each of the LCD pixels according to addressing electrical signals so as to transform the electrical signals into spatial light intensity distribution, and F in FIG. 1 represents a focal length of Fourier lens. It is possible to display a dynamic computer generated hologram by using such a liquid crystal spatial light modulator as recording media instead of a photographic film. However, the method needs a very high pixel resolution, it has to be based on silicon based liquid crystal fabrication, and has a relatively high cost, making it not easy to fabricate a large area computer generated hologram.
In summary, when using an optical microphotography system or micro optical system to display a computer generated hologram in conventional technology, it is possible to display a static computer generated hologram only and the process is complex. When a liquid crystal spatial light modulator is used to display a computer generated hologram, it is possible to display a dynamic computer generated hologram, however the fabrication cost is relatively high and it is not easy to fabricate a large area computer generated hologram.